Capacitive measuring head



y 22, 1952 D. B. PARKINSON ETAL 2,604,513

CAPACITIVE MEASURING HEAD Filed July 25, 1951 F'IG.3

' CAPACITANCE MEASURING DEVICE INVENTOR. DAVID B. PARKINSON CHARLES BIELECKI ApoR NEY Patented July 22, 1952 CAPACITIVE MEASURING HEAD David B. -Parkinson, Cleveland Heights, and Charles Bielcclii, Parma, Ohio, assignors to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application July 25, 1951, Serial No. 238,408

s Claims. (01. 175-183) The present invention relates, in general, to a capacitive measuring head for testing materials by the measurement of a capacitive eliect of the material being tested and relates partic- Furthermore, since the capacitance variations to be measured are so very small, it is very difficult to provide a testing head for the purposes under consideration which is sufiiciently stable ularly to such a head which is used for the test- 5 mechanically or sufficiently stable with teming of yarns, roving, and sliver in textile manupcrature variations that such changes are not lacturing processes. of the same order of magnitude as those to be Various prior-art processes have utilized demeasured in testing the material. vices for measuring the capacitive eiiect of a Applicants have provided a capacitive measmaterial in order to test some quality or characuring head for testing materials by the measteristic of the material. For example, it has urement of a capacitive effect of the material been proposed to use such measurements to test being tested which eliminates or reduces to a the uniformity of yarns, roving, and sliver in minimum the clifliculties mentioned above. the manufacturing of textiles. By such meas- It is an object of the invention, therefore, to urements, the ni rmi y o e yarns, roving, 5 provide an improved capacitive measuring head or sliver can be measured or recorded and, in for testing materials by the measurement of a recent years, some such devices have come into capacitive elfect of the materials being tested. a considerableextent of use. In such arran e It is still another object of the invention to ments, it has been the practice to cause the maprovide an improved capacitive measuring head terial being tested to run between the plates of which is very rigid mechanically and in which a cond ns r and the c p i n of th concapacitive variations with temperature are redenser, or variations in capacitance thereof, have du ed to a, minimum. been measured and used to ca the 6- It is still another object of the invention to sired characteristic of the material. However, r vide a capacitive measuring head for the since the condenser involve is on havi an testing of materials by the measurement of a air dielectric and, inasmuch as the dielectric capacitive effect of the materials being tested constant of materials of he yp n er c n idwhich is interchangeable with other heads haveration which are to be tested is very close to ing air gaps of different size and in which the that of air, it is ne e y o m s r in ch adjustments to an instrument indicating cadev ces Ve y Small changes in capacitance- For pacitance variations of the material being tested this reason. it has been found n c s ry to fix are eliminated, or made very simple, and a null the Sp between the co d s r plates to proreading on the capacitance measuring device is vide the best results for the material t0 be provided in all cases where the material being tested. Since the thickness of gauge of such measured i not present. materials of d fe e t yp a es Over a ela- For a better understanding of the present intively Wide extent, it is frequently eces ary t vention, together with other and further obp ov d in testing dvices 0f the yp h e unjects thereof, reference is bad to the following der consideration, a plurality of difierent measdescription taken in connection with the accomurln a s havi a r gaps of sev l d fe Danying drawings, and its scope will be pointed dimensions. However, since the capacitance 40 out in the appended claims. measurements being made are so very critical, Fig. 1 of the drawings illustrates the general great difficulty is encountered in making an artype of equipment towhich the present invenrangement which has identicalcharacteristics tion pertains; Fig. 2 illustrates a capacitive with an assortment of mea u heads having measuring head constructed in accordancewith air gaps of different sizes. Particularly, it is the invention; and Fig. 3 is an exploded view of desired that the indicating device-associated with the various parts which go into the assembly the capacitance measuring equipment indicate of the Fig. 2 arrangement. a zero value in the absence of any material be- Referring now more particularly to Fig. 1, ing tested, even though the measuring headsused there is shown a measuring head H) which can with the equipment have air gaps which vary be used for the testing of materials by the measover a rather wide range of values. If this is urement of a capacitive eifect of the material not done, it is necessary to make precise adbeing tested. The device it) effectively comjustments to; the equipment each time the air prises two condenser plates and the material to gap is changed in order to provide this zero be tested is placed between these condenser reading, 66 plates and the variation of capacitance when 3 the material is introduced between the condenser plates, or as various lengths of the material are successively drawn through the condenser plates, is used as an indication of the quality of the material. The capacitance of the condenser may be measured by any suitable arrangement such as the capacitive measuring device II. The measuring device has associated therewith a meter [2 which has a zero reading in the absence of the presence of a material to be tested between the plates of the condenser.

Since, as mentioned above, the capacitance changes in a system of the type here under consideration are generally very small, it is necessary generally to shield the measuring condenser from external effects; for example, by means of a shield as indicated by the numeral [4 of Fig. 1. Also, it is frequently desirable that the plates of the condenser be electrically balanced with reference to ground and such an arrangement is provided by many of the prior-art capacitance measuring devices.

Applicants capacitive measuring head is illustrated in Fig. 2 and effectively comprises two very rigid conducting housings l6, [6 which are spaced by a heavy metal spacer I! and held together by means of screws l8, Hi to provide an air gap l9 into which the material to be tested is placed. Applicants measuring head is readily interchangeable with other measuring heads having different gap widths in that prongs 20, are provided for plugging the measuring head into an electrical circuit. Reference is made to Fig. 3 for a more detailed description of the measuring head of applicants invention.

Fig. 3 is an exploded view of all of the parts which are contained in the device of Fig. l. Elements in Fig. 3 which are similar to those of Fig. 1 bear identical reference numerals. The electrodes illustrated in Fig. 2 are of relatively heavy construction and one of these is illustrated in Fig. 3 by the reference numeral 22. This electrode is preferably of metal and has an operating face 23 and other surfaces which have a combined area much greater than the area of the operating face. In order to hold the electrode very rigidly in position, the electrode 22 is placed in the housing (6 and held in position by means of a plastic material, for example of the thermosetting type, the plastic material being poured around the electrode 22 while it is in position in the housing It. After the plastic sets, it has a shape as represented by the element 24 of Fig. 3, although it will be understood that it is physically impossible to remove the plastic material from the housing [6 after the plastic material has set and that it is shown separately in Fig. 3 solely for purposes of illustration. This element is preferably ofa low-loss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which materials are to be tested in the measuring head of the invention. In order to provide a capacitance adjustment between the electrode 22 and the housing It, a trimmer condenser 25 is provided. This condenser effectively comprises a metal slug 26, the position of which can be moved by means of a threaded member 21. The slug 26 is enclosed in a glass shell 28 and metal member 29 is provided at one end thereof and a threaded cap 30 is provided at the other end thereof. The member 29 is provided with an electrical lead 3|.

'In assembling the portions of the Fig. 3 arrangement which have so far been described, the

procedure is as follows: The trimmer condenser 25 is placed inside the metal housing It and the threaded member 21 is brought out through the hole 32. The electrode 22 is then placed in the metal shell I5. A nut is threaded from outside the housing l6 onto the threaded member 30 thus effectively holding the trimmer condenser in place. The electrode 22 is next placed in the housing IS with the channel portion 33 thereof effectively surrounding the trimmer condenser 25. The plug 20 is next screwed into a tapped hole 34 in electrode 22. The dielectric material to be utilized is then poured into the casing it around the electrode 22 and allowed to harden.

The assembly of the other electrode of the device 22' in its dielectric material 24' is also illustrated in Fig. 3. Other portions of the remaining housing, electrode and dielectric material which are similar to those which have been described in detail, have similar reference numerals primed. The lead 3| is illustrated as being affixed to the electrode 22', as by soldering.

It will be understood that the two halves of the measuring head of Fig. 3 are in all respects similar. As so far described then, the arrangement of Fig. 3 comprises the two casings l6, l6, each of which has therein the block of dielectric material 24 or 24 and these blocks of dielectric material, respectively, hold in position the electrodes 22 and 22.

The whole arrangement is next assembled by means of screws I8, [8 and by means of the spacer I! to provide a completed measuring head in accordance with the invention.

The elements 24 and 24 are preferably of a low-loss dielectric material which has a dielectric constant which is relatively stable with temperature over the range at which materials are to be tested in the apparatus. This dielectric material should also be mechanically stable with temperature changes, and it has been found that some polystyrenes are very satisfactory for the intended purpose. In one preferred embodiment of the invention, applicants have used a material made in accordance with the following formula and its use has been found to be very satisfactory:

Ingredients by weight:

Clear polystyrene granules, approx. 62%

Styrene monomer, approx. 32% (25 to 35) Hydrogenated terphenyl, approx. 5% (2 to -8) Di-vinyl benzine (solution), approx.

(1-H; to 1) Benzoyl peroxide-catalyst, approx.

The actual percentages are not particularly critical. Curing takes place in an oven at relatively low temperature (70 C.) and is accomplished in 24 hours. Higher temperatures will speed up the curing process, but there is greater danger of evaporation and consequent deformation. Polymerization molding, as opposed to injection molding is less destructive to the insert trimmer capacitor and the positioning of the electrode.

The electrical losses in this material are very small, the material having a power factor of 0.0003 at'7 megacycles and having a dielectric constant of about 2.55.

It will also be seen that the metal housings l6, l6, substantially completely surround the electrodes 22, 22, resspectively. This has the eifect of providing an electrostatic shield which performs the function of shield H of Fig. 1. It will furthermore be seen that the member I! efiecas between the various heads.

tively comprises a metal shim and that this shim can be machined very precisely to the required gap width for the measuring head before it is inserted in the structure.

is necessary in order for the capacitance measuring device, which may be of the nature of the element ll of Fig. 1, to read zero for any ofthe various heads which may be used, to have the capacitance between electrodes 22, 22 adjustable In other words each measuring head used with the capacitance measuring device H must present thereto the same-value of capacitance in the absence of a material to be tested. It is for this reason,

among others, that the trimmer condensers 25, -25 are provided.

In a preferred embodiment of the invention, the trimmer condensers have a capacitance which is adjustable over the range from20 to micro microfarads. Also, in providing an'i'nstrument which is suitable for the testing of most yarns, roving and sliver, it is desirable to provide an arrangement which, for the various shims ll utilized, has a capacitance at terminals 20, 20 which, is within the range of 0.3 to 3.0 micro microfarads.

What is claimed is: I c 1. A capacitive measuring head for testing materials by the measurement of a capacitive effect of the. material being tested comprising: two conductive electrodes, each having a relatively fiat operating face; two elements of lowloss dielectric material which-has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode substantially exposed; means for rigidly mounting said housings with said electrode faces in an opposed fixed stable relationship and with a conductive connection between said housings; means extending through each said housing for connecting an external circuit to said electrodes; and a separate trimmer condenser between each said housing and its enclosed electrode.

2. A capacitive measuring head for testing materials by the measurement of a capacitive effect of the material being tested comprising: two conductive electrodes, each having a relatively flat operating face and other surfaces which have a combined area much greater than that of the area of said operating face; two elements of lowloss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode exposed; means for rigidly mounting said housings with said electrode faces in an opposed fixed stable relationship and with a conductive connection between said housings; means extending through each said housing for connecting an external circuit to said electrodes;

and a separate trimmer condenser between each said housing and its enclosed electrode. 3. A capacitive measuring head for testingmaterials by the measurement of a capacitive effect of the'material being tested comprising: two conductive electrodes, each having arelatively flat operating face; two structurally unitary elements of low-loss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to --be tested, each of said elements substantially surrounding one of said electrodes'except for its operating face;

two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode exposed; means for rigidly mounting said housings with said electrode faces in an-oppcsedfixed stable relationship and with a conductive connection between said housings; means extending'through each said housing for connecting an external circuit to said electrodes;

and a separate trimmer condenser between each said housing and its enclosed electrode.

4. Acapacitive measuring head for testing materials by the measurement of a-capacitive effect of the materialbeing tested comprising: we conductive electrodes, each having a relatively'fia't operating face; two elements of low-loss dielecments substantially surrounding one of said electrodes except for its operating face and being substantially. flush with said operating face; two

rigid conducting housings, each substantially enclosing one of. said-elements and its enclosed electrode and being substantially flush with said operating face of said enclosed electrode but leaving said face of said enclosed electrode exposed; means for rigidly mounting said housings with said electrode faces in an opposed fixed relationship and with a conductive connection between said housings; means extending through said housing for connecting an external circuit to said electrodes; and a separate trimmer condenser between each said housing and its enclosed electrode.

5. A capacitive measuring head for testing materials by the measurement of a capacitive effect of the material being tested comprising: two conductive electrodes, each having a relatively flat operating face; two elements of low-loss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed elec trode but leaving said face of said enclosed electrode exposed; means including a dimensionally stable spacer for rigidly mounting said housings with said electrode faces in an opposed fixed relationship and with a conductive connection between said housings; means extending through each said housing for connecting an external circuit to said electrodes; and a separate trimmer condenser between each said housing and its enclosed electrode.

6. A capacitive measuring head for testing materials by the measurement of a capacitive effect of the material being tested comprising: two conductive electrodes, ,each having a relatively flat operating face; two elements of low -loss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode exposed; means including a conductive spacer for rigidly mounting said housings with said electrode faces in an opposed fixed relationship and with a conductive connection between said housings; means extending through said housing for connecting an external circuit to said electrodes; and a separate trimmer condenser between each said housing and its enclosed electrode.

7. A capacitive measuring head for testing materials by the measurement of a capacitive efiect of the material being tested comprising: two conductive electrodes, each having a relatively vfiat operating face; two elements of low-loss dielectric material which has a dielectric constant which is relatively. stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode exposed; means for rigidly mounting said housings with said electrode faces in an opposed fixed stable relationship and with a conductive connection between said housings; means extively stable over said temperature range between each said housing and its enclosed electrode.

8. A capacitive measuring head for testing materials by the measurement of a capacitive effect of the material being tested comprising; two conductive electrodes, each having a relatively flat operating face; two elements of low-loss dielectric material which has a dielectric constant which is relatively stable with temperature changes over the temperature range at which said materials are to be tested, each of said elements substantially surrounding one of said electrodes except for its operating face; two rigid conducting housings, each substantially enclosing one of said elements and its enclosed electrode but leaving said face of said enclosed electrode exposed; means for rigidly mountingsaid housings with said electrode faces in an opposed fixed stable relationship and with a conductive connection between said housings; and detachable mounting means extending through each said housing for detachably connecting an external circuit to said electrodes; and a separate trimmer condenser between each said housing and its enclosed electrode.

DAVID B. PARKINSON. CHARLES BIELECKL- REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS Number Name Date 2,535,027 Anderson Dec. 26, 1950 2,555,977 Kline June 5, 1951 

